Global Navigation Satellite Systems (GNSS) include the Global Positioning System (GPS), Glonass, and the proposed Galileo system.
FIG. 1 illustrates a typical prior-art problem with GNSS signal reception. The antenna of a GNSS receiver such as antenna 105 is disposed for receiving signals from satellites such as satellites 3, 7, 8, 10, 12, 22. Reception of signals from satellites in line-of-sight view of antenna 105 is generally good. Signals received from satellites blocked by obstructions such as building 110 and tree canopy 115 are attenuated and distorted. These obstructions can also cause signal refraction and multipath reception at the antenna due to reflections within the canopy. Distorted signals degrade the accuracy of GNSS fixes such as fixes of position, time and velocity determined from the received signals.
Various techniques are used to reduce multipath reception from typical reflectors. GPS antenna assemblies are known having choke ring devices which attenuate signals arriving at the antenna from low angles near the horizon or even from angles below the plane of the antenna. It is known for GPS antennas to employ circularly polarized antenna elements which may attenuate a multipath signal if the reflection truly reverses the polarization. GPS receivers also use software algorithms to attenuate multipath signals. Such techniques have not been found effective in the case of signals passing through or near typical obstructions found in field environments such as canopy in which the signal distortions do not follow predicted models defining multipath behavior.
Obstructions passing through the field of view of the GPS antenna may affect only a subset of the available GPS satellite signals. However, these aforementioned methods of multipath mitigation are not satisfactory since they cannot identify specific GPS satellite signals which are affected by passing obstructions, nor can they identify specific GPS satellite signals which are not affected by passing obstructions.
Methods and apparatus for improved selection of GNSS satellite signals are needed.